Iraqi forces faced stiff resistance from the Islamic State (IS) group in northwest Mosul on Sunday after opening a new front against the militants in a push to rout them from the city after seven months of fighting.

Trapped in a steadily shrinking area of the city, the militants are fighting back with a barrage of suicide car bombs and snipers concealed among hundreds of thousands of civilians they are effectively holding hostage.

Having gained a foothold in the northern Musherfa district last week, Iraqi forces are trying to push down into the handful of remaining districts held by IS in Mosul, including the Old City.

The Iraqi military said on Saturday the Musherfa district had been fully retaken, but officers told Reuters on Sunday there was still fighting there and smoke could be seen rising over the area by a Reuters reporter in east Mosul.

"Fighting Daesh in Musherfa with a large number of families still locked in their houses is making the battle more complicated," said a colonel from the ninth armoured division, which is taking part in the offensive, referring to IS by its Arab acronym.

The new push from the northwest began last week after other fronts in the city's southern districts stalled around the Old City where near the iconic mosque from which the IS leader proclaimed a modern-day caliphate.

Iraqi forces are backed by a US-led coalition that is carrying out air strikes and advising them on the ground.

Iraqi officers said additional reinforcements from the Rapid Response Division had arrived in northwest Mosul on Sunday to help clear areas on the banks of the River Tigris that bisects the city.

"The brigade will participate in the battle to retake Hawi al-Kanisa and also help the ninth army division step up pressure on Daesh fighters," said a Rapid Response officer on condition of anonymity.